Preliminary Bulk Density Mapping of the Upper and Lower Bakken Member Shales of Southeastern Saskatchewan: A Potential Indicator for Oil Generation and Expulsion Dan Kohlruss 1 and Kosta Stamatinos 2 Kohlruss, D. and Stamatinos, K. (2014): Preliminary bulk density mapping of the Upper and Lower Bakken Member shales of southeastern Saskatchewan: a potential indicator for oil generation and expulsion; in Summary of Investigations 2014, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of the Economy, Misc. Rep. 2014-4.1, Paper A-2, 7p. Abstract The use of bulk density coupled with formation resistivity of shale source rocks is a possible method of identifying the level of maturity of the rocks, and their potential for oil generation and oil expulsion. The Late Devonian to Early Mississippian Bakken Formation shales of the Williston Basin are extremely rich source rocks and in some areas of the basin have been responsible for generating and expelling vast amounts of oil. Migration of this oil into suitable stratigraphic traps has resulted in unprecedented economic gain in parts of Saskatchewan, Manitoba, North Dakota and Montana. Studies of the Bakken Formation in central Williston Basin in North Dakota have identified areas where its shales are, without question, thermally mature and have generated and expelled oil. However, Bakken Formation shales in northeastern Williston Basin in Saskatchewan have been examined and reported as being thermally immature, with no evidence of oil generation or expulsion. This paper will illustrate how mapping bulk density and formation resistivity has identified anomalous areas in southeast Saskatchewan where Bakken Formation shale source rocks may indeed be thermally mature and may indeed have expelled oil, contrary to previous studies’ findings. Keywords: Bakken Formation, source rocks, tight oil, bulk density, formation resistivity, Late Devonian, Early Mississippian, southeast Saskatchewan. 1. Introduction Southeastern Saskatchewan’s tight oil (low permeability) Bakken oil play has become an astonishing success story. Favourable economic conditions have enabled the use of horizontal wells combined with multistage fracturing to liberate oil from the Bakken’s extremely low permeability (<1 millidarcy (mD)) reservoir rocks in the Viewfield, Ryerson and Roche Percee oil pools (Figure 1). As of November 30, 2014 there were 2591 producing Bakken Formation oil wells in southeast Saskatchewan. Since 1956, Bakken wells have cumulatively produced 25.8 million cubic metres (m3) of oil at a current rate of 9754 m3/day. The majority of this production has occurred since 2005 and has been from the Viewfield oil pool. An emerging Bakken Formation and Torquay Formation (Bakken-sourced oil) oil play along the Saskatchewan– North Dakota–Montana border (Figure 1) has been of interest recently because it offers new opportunities for oil production from the Bakken. This emerging oil play is likely an extension of the Bakken shale oil play in North Dakota, in central Williston Basin, which is reliant on close vertical proximity of thermally mature Bakken shale source rocks and juxtaposed reservoir rocks, including Middle Bakken siltstones and sandstones as well as the Torquay’s silty dolostones. The location of the new production in Saskatchewan supports a theory that mature Bakken source rocks may extend farther north than previously thought. The purpose of this paper is to use non-standard techniques of bulk density and formation resistivity mapping to strengthen the theory that primary migration of oil from Upper and Lower Bakken shales into Middle Bakken siltstones and sandstones and Torquay Formation silty dolostones has occurred north of the Canada–US international border. 1 Saskatchewan Ministry of the Economy, Saskatchewan Geological Survey, 201 Dewdney Avenue East, Regina, SK S4N 4G3. 2 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2. Saskatchewan Geological Survey 1 Summary of Investigations 2014, Volume 1 Figure 1 – General location of the study area. The green polygons represent existing Bakken shale oil pools, while the region in red represents new Bakken Formation and Torquay Formation tight oil production along the Canada–US international border. Oil produced from the Torquay Formation is Bakken-sourced oil. Inset shows study area in the context of the location of the Williston Basin (thin black outline) and the Bakken Formation (Exshaw Formation in Alberta; area coloured in green). Abbreviations on inset: AB = Alberta, SASK = Saskatchewan, MAN = Manitoba, MT = Montana, ND = North Dakota. 2. Study Area The study area is bounded to the east by the Saskatchewan–Manitoba border, to the south by the Saskatchewan–US (North Dakota and Montana) international border, to the west by the Third Meridian (W3) and to the north by the northern boundary of Township 12 (Figure 1). The study area includes all Saskatchewan’s tight oil Bakken production. For the purposes of this paper, tight oil is defined as reservoirs with low permeability (<1 mD) that require horizontal drilling and multistage fracturing to stimulate production. 3. General Stratigraphy The Three Forks Group Bakken Formation is comprised of siltstone and sandstone (Middle Bakken Member) between upper and lower black, organic-rich shales (Upper and Lower Bakken members; Figure 2). The Bakken Formation overlies argillaceous siltstones and silty dolostones of the Devonian Torquay and Big Valley formations. Where present, the underlying Big Valley Formation is unconformable with the Lower Bakken Member shale (Christopher, 1961). Where the Torquay underlies the Bakken, the contact at the base of the Lower Bakken is relatively sharp and also unconformable. In the eastern portion of the study area (approximately Ranges 30 to 33W1M), the Lower Bakken and lowermost portion of the Middle Bakken are absent and the Middle Bakken is in sharp contact with Torquay Formation silty dolostones (Nickel, 2010). The Bakken Formation is overlain by organic-rich fossiliferous limestones of the Souris Valley Beds (Lodgepole Formation) of the Madison Group, which conformably overlie the Upper Bakken Member shale (Christopher, 1961). Saskatchewan Geological Survey 2 Summary of Investigations 2014, Volume 1 Figure 2 – Stratigraphic chart of the Devonian– Mississippian units within the Saskatchewan, Three Forks and Madison groups in southeast Saskatchewan (modified from Nickel, 2010). 4. Source Rock, Thermal Maturity and Primary versus Secondary Oil Migration in the Bakken Formation of Saskatchewan Oil in Saskatchewan’s Viewfield pool has been sourced from the Upper and Lower Bakken shales. The oil is not locally sourced, however (i.e., not primary), but has migrated up-dip from central Williston Basin in North Dakota, where the shales are known to be thermally mature and the majority of oil pools are a result of primary oil migration. The oil travelled northward into Saskatchewan and was stratigraphically trapped where the Middle Bakken Member reservoir rocks pinch out. The Bakken Upper and Lower shale members in Canada are comprised of highly organic, black, carbonaceous and pyritic shales with total organic carbon (TOC) contents averaging slightly less than 12% in the Lower Member and nearly 18% in the Upper Member (Kreis et al., 2006). Although areas in North Dakota have been identified as having thermally mature source rocks, it has been reported that the Upper and Lower Bakken Member shales are not thermally mature anywhere in Canada (Osadetz et al., 1992; Osadetz and Snowdon, 1995). The indicators of thermal maturity—Tmax and hydrogen index—reported for the Bakken shale in Saskatchewan have been found to be too low and too high, respectively, suggesting that Bakken shales in Saskatchewan have not generated or expelled oil. Drilling and production from the Bakken and Torquay formations along the Saskatchewan–US international border suggests otherwise, and perhaps unorthodox oil exploration techniques such as using shale resistivity and/or shale bulk density mapping could reveal possible thermal maturity in these rocks. a) Shale Resistivity and Oil Generation Several authors (e.g., Meissner, 1978; Schmoker and Hester, 1990; Kreis et al., 2006) have linked an increase in shale resistivity to either migrated (secondary) or generated (primary) oil within the Bakken Formation. In particular, the research undertaken by Schmoker and Hester (1990) found that resistivity values greater than 35 ohm-m in US Bakken shales correspond with the commencement of oil generation. b) Shale Bulk Density and Oil Maturity According to Jarvie et al. (2011) immature Bakken shales, like those found in Saskatchewan, are partially kerogen- supported, based on their high TOC, and subsequently have very low bulk density (< 2.00 to 2.10 grams per cubic centimetre (g/cc)). They state that, as the kerogen in the shale matures (that is, as increased burial depth results in increased temperature and pressure), it becomes pliable and loses strength. This will cause the kerogen to begin to convert to bitumen and oil. The development of pressure from this process can induce microfractures, and oil is expelled from the thermally weakened kerogen through these fractures. Eventually the kerogen compacts, resulting in Saskatchewan Geological Survey 3 Summary of Investigations 2014, Volume 1 an increase in the shale’s bulk density and an increase in oil content as the source rocks become progressively more mature. Jarvie et al.’s (2011) work